In order to charge a battery as rapidly as possible whilst simultaneously ensuring that the lifetime of the battery is prolonged, correct charging comprises a number of phases. During the bulk charging phase the charger is usually operating at its current limit and the voltage across the battery terminals increases as the battery is charged. At the end of the bulk charging phase the battery will hold a high proportionate charge, for example 80% of the maximum. When the bulk charging phase is completed the absorption charging phase begins, during this phase voltage is held constant at a comparatively high level which is achieved through regulation of the current. The transition between bulk charging and absorption charging can be determined by measuring the variation in the battery voltage. Overly prolonged absorption phase charging can be damaging to the battery, hence before damage can occur and typically before the battery is fully charged it is necessary to move to the float phase. During the float phase the voltage is reduced to a level where damage to the battery cannot occur, irrespective of how long the float phase lasts. Regulation of the current in both absorption and float phases also ensures that the battery does not overheat, thereby preventing any unwanted consequences for the longevity of the battery. At the end of a sufficiently long float phase the battery should be fully charged.
It is preferable for some battery types to continue charging until the battery is fully charged, so that the battery lifetime is prolonged. In the mobile radio base site power supply application it is desirable to identify the point at which the battery is fully charged, so that the generator can be switched off at the earliest opportunity in order to conserve fuel. Accurate determination of battery state of charge can enable the charging cycle to be completed in the absorption charging phase so that it is not necessary to go into a float phase. The state of charge value can also be used to trigger the activation of the generator to prevent the battery from being discharged too much, with the possible adverse consequences for battery longevity which would otherwise occur.
The state of charge of a battery can be determined by making measurements of battery voltage, however, this approach can be problematic for a number of reasons. One problem is that in some charging phases such as absorption there is little variation in voltage over a wide range of state of charge, which can lead to inaccurate estimation of state of charge. This can be problematic in situations, where it is necessary to configure the system to operate within a state of charge range for which the change in battery voltage is marginal, which might be the case for example when needing to prolong battery life. Another problem with voltage measurement based state of charge estimation is that battery voltage will be affected by battery temperature and the magnitude of the current which is being drawn, both of which would need to be compensated for.
An alternative method for determining battery state of charge is to monitor the charge that is flowing into and out of the battery in order to determine the state of charge of the battery at any particular moment, this method is called Coulomb counting. Using this technique it is possible to maintain an accurate estimate for the state of charge over all charging phases. The Coulomb counting approach also has a number of problems. One problem is that a method is needed for initializing the value to be used in the Coulomb count when the current state of charge of the battery is unknown, for example when a new battery is first connected to the charge management system. Another problem is that the Coulomb count may drift from the true value over time due, for example to measurement inaccuracy or internal discharge of the battery. A final problem is that the Coulomb count will give an inaccurate value for the percentage state of charge of the battery if the capacity of the battery has changed for any reason.
In a further aspect, in order to charge a battery as rapidly as possible whilst simultaneously ensuring that the lifetime of the battery is prolonged, correct charging comprises a number of phases. During the bulk charging phase the charger is usually operating at its current limit and the voltage across the battery terminals increases as the battery is charged. At the end of the bulk charging phase the battery will hold a high proportionate charge, for example 80% of the maximum. When the bulk charging phase is completed the absorption charging phase begins, during this phase voltage is held constant at a comparatively high level. The transition between bulk charging and absorption charging can be triggered when the measured battery voltage during bulk charging reaches a pre-determined threshold level. Overly prolonged absorption phase charging can be damaging to the battery, hence before damage can occur and typically before the battery is fully charged it is necessary to move to the float phase. During the float phase the voltage is reduced to a level where damage to the battery cannot occur, irrespective of how long the float phase lasts. Regulation of the current in the float phase may also be used to ensure that the battery does not overheat, thereby preventing any unwanted consequences for the longevity of the battery. At the end of a sufficiently long float phase the battery should be fully charged.
It is preferable for some battery types to continue charging until the battery is fully charged, so that the battery lifetime is prolonged. In the case of a diesel generator/battery hybrid power supply it is desirable to identify the point at which the battery approaches or reaches full charge, so that the generator can be switched off at the earliest opportunity in order to conserve fuel. Accurate determination of battery state of charge can enable the charging cycle to be completed in the absorption charging phase so that it is not necessary to go into a float phase. The state of charge value can also be used to trigger the activation of the generator to prevent the battery from being discharged too much, with the possible adverse consequences for battery longevity which would otherwise occur.
The state of charge of a battery can be determined by making measurements of battery voltage, however, this approach can be problematic for a number of reasons. One problem is that in some charging phases such as absorption the voltage level is regulated to a fixed level so that it cannot be used to determine state of charge. This can be problematic in situations where the system operates within a state of charge range corresponding to the absorption phase, for example when needing to prolong battery life or when availability of power from unreliable sources such as renewables is good. Another problem with voltage measurement based state of charge estimation is that battery voltage will be affected by battery temperature which would need to be compensated for.
An alternative method for determining battery state of charge is to monitor the charge that is flowing into and out of the battery in order to determine the state of charge of the battery at any particular moment; this method is called Coulomb counting. Using this technique it is possible to maintain an accurate estimate for the state of charge over all charging phases. The Coulomb counting approach also has a number of problems. One problem is that a method is needed for initialising the value to be used in the Coulomb count when the current state of charge of the battery is unknown, for example when a new battery is first connected to the charge management system. Another problem is that the Coulomb count may drift from the true value over time due, for example to measurement inaccuracy or internal discharge of the battery. A final problem is that the Coulomb count will give an inaccurate value for the percentage state of charge of the battery if the capacity of the battery has changed for any reason.